In order to adhere to increasingly stricter exhaust gas specifications, most motor vehicles are nowadays equipped with an exhaust gas purifying catalytic converter. However, the optimal purification efficiency of the catalytic converter is only achieved in the case of a specific air/fuel ratio of the induction mixture. In order to adjust the air/fuel ratio, at least one lambda sensor is arranged in the exhaust gas duct of the motor vehicle. However, lambda sensors have a number of disadvantages.
Therefore, a reliable signal of the lambda sensor can only be obtained after the operating temperature of approximately 800 to 900° C. has been reached. As a result, the lambda sensor must, after starting of the internal combustion engine, be heated to this operating temperature by means of a heating device that has specially been provided. Therefore it is impossible to control the air/fuel ratio until the operating temperature has been reached, which leads to an increased discharge of harmful substances during the running-up phase.
Furthermore, when using inexpensive, binary lambda sensors, the determination of a defined lambda value of the exhaust gas is not possible because a binary sensor only allows a qualitative statement about the composition of the exhaust gas. Although linear lambda sensors do allow a quantitative determination of the combustion lambda value, they are far more expensive than binary lambda sensors.
In the case of motor vehicles which can optionally be operated by means of different types of fuel such as for example gasoline and alcohol, the combustion lambda value can, as a rule, only be determined by means of a lambda sensor during operation with one of the two fuel types.
As explained above, the use of lambda sensors is subject to strict restrictions.